Back in the day Britain had its own version of the beer purity law, which only allowed malt to be used for providing the fermentables when brewing. The law wasn't brought in due to any concerns about beer quality or consumer safety - it was because tax was paid on malt.

Poor harvests made Parliament temporarily relax the law and allow sugar to be used up replace up to 25% of the malt in the years 1800 and 1812-13. In 1847 the law was changed permanently so that sugar was now allowed, though customs duty and/or taxes ensured that the equivalent taxation to malt was levied on it.

At first take up of sugar use was slow and up until 1866 it was never over 10 million pounds. After this time, due to a reduction in cost and sugar refiners producing better products use rose sharply so in 1879-80 over 127 million pounds were used.

In 1880 tax on both sugar and malt were abolished, and the mash tun was "freed" so that brewers were no longer restricted on what they could use to provide fermentable material. They weren't freed from taxation though, the goverment had shifted it to beer.

On the use of sugar Patton states:

"Initially sugar was merely seen as a replacemnt for malt, but the effect it had on the flavour of beers led brewers to introduce new beers in which the luscious flavour of the sugar was a more prominent feature. Primings, though they were introduced to give a quick maturation to running beers, also imparted their luscious flavour to the beer. As their use increased, running beers generally became more sweet and it accentuated the difference between the runing beers and the stock beers, probably hastening the decline of the latter."

This gives me evidence for my current theory on why invert sugar was popular with brewers. Invert sugar (glucose/fructose mix) is sweeter than the sucrose from which it is made, and people liked the taste.

I'm still dubious about the line that producing the invertase needed to break sucrose down to glucose and fructose stresses the yeast. In 1960 bulk liquid sugar, which is mainly sucrose, became more popular than invert sugar, and as far as I can see it didn't cause any problems for the breweries. And of course they benefitted from the tax advantage of hydrolysis gain.

When Patton talks of priming sugar he makes clear it carries out a dual role:

"1. To give sweetness and flavour.
2. To provide fermentable sugar for a secondary fermentation"

He continues:

"They were first used at the end of the nineteenth century in weaker running beers, which did not contain as much residual sugars as the stronger beers. The priming sugars were fermented rapidly and enabled the beers to mature quickly. Initally the amount of priming sugar was limited to 1.5% of the volume of the beer and had to have a gravity no higher than 1150. There was some feeling at the time that when priming sugars were added to a beer, they gave the impression that the beer was much stronger than the gravity to which the primings had raised it and that because of this revenue could be lost ... A standard mixture which does not crystalise out is 55% invert sugar and 45% cane sugar"

As it's now more expensive to get fermentable extract from sugar than it is from malt the use of sugar in brewing, inverted or not, has declined greatly. Ragus, the last remaining manufacturer of invert sugar blocks, says it now accounts for just 2.5% of their sales.